-The ability to backup your games and play them from a USB device.
-Turn your Wii in a Media Center (Play Movies, Music and more from USB, SD or DVD*)
-The ability to play games from other regions (full region free)
-The ability to use dozens of Homebrew Applications
-Custom themes for the System menu.
-The ability to move your disc channel.
-Use emulators to play various games from the past.
-Play game hacks, such as the Brawl mods and texture hacks.
-None that we can think of
See here for more information